Thats teh operitive word. Cable modems are shared bandwidth. So its almost impossible for your cable company to provide consistant service to your household unless they truly cap download speeds for everyone and make sure they dont overload the subscribers on any given neighborhood feeder.
Another thing to remember is the conversion of bits to bytes. 8 bits in a byte and bandwidth is often measured in bits where as disk transfer is measured in bytes.
So if your downloading and you see IE report 8KB/sec then your really transfering at 64kb/sec. Notice the KB vs kb speeds.
There is always some level of overhead to transfer that is needed for what I call control packets. These are packets that fly back and forth confirming your system got the data it was supposed to get or requesting the data be sent again, etc.